ABSTRACT: Using radiative transfer theory I show that for a given waveband, the total radiant energy, Qt, present within the water column (per square meter of surface) is proportional to the scalar irradiance just below the surface [E0(0)] and inversely proportional to the irradiance-weighted vertical attenuation coefficient for scalar irradiance (wK0,av), in accordance with[eqn] where cw is the speed of light in the water. Further, Qt = E(0)/cwa where E(0)is the net downward irradiance just below the surface, and a is the absorption coefficient of the water for the waveband in question. The average depth, zp, of all the photons in the water column in a given waveband is equal to [wKE,av]-1, the reciprocal of the irradiance-weighted vertical attenuation coefficient for net downward irradiance. This conclusion, at which we arrive on theoretical grounds, is confirmed by numerical modeling of the underwater light field.

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